Stable Operation and Evaluation of Transimpedance Amplifier

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Stable Operation and Evaluation of Transimpedance Amplifier [Copy link]

A circuit that can automatically convert DC power into an AC signal with a certain amplitude and frequency without an external signal is called an oscillation circuit or oscillator. This phenomenon is also called self-excited oscillation. In other words, a circuit that can generate an AC signal is called an oscillation circuit.

An oscillator must include three parts: an amplifier, a positive feedback circuit, and a frequency selection network. The amplifier can amplify the input signal applied to the oscillator input terminal so that the output signal maintains a constant value. The positive feedback circuit ensures that the feedback signal provided to the oscillator input terminal is in the same phase, only in this way can the oscillation be maintained. The frequency selection network only allows a specific frequency f 0 to pass through, so that the oscillator produces a single frequency output.

Whether the oscillator can oscillate and maintain a stable output is determined by the following two conditions; one is that the feedback voltage uf and the input voltage U i must be equal, which is the amplitude balance condition. The second is that uf and ui must have the same phase, which is the phase balance condition, that is, it must be guaranteed to be positive feedback. In general, the amplitude balance condition is often easy to achieve, so when judging whether an oscillation circuit can oscillate, it mainly depends on whether its phase balance condition is established.

Oscillators can be divided into ultra-low frequency (below 20 Hz), low frequency (20 Hz ~ 200 kHz), high frequency (200 kHz ~ 30 MHz) and ultra-high frequency (10 MHz ~ 350 MHz) according to the oscillation frequency. Oscillators can be divided into two categories according to the oscillation waveform: sinusoidal oscillation and non-sinusoidal oscillation.

Sine wave oscillators can be divided into three types according to the components used in the frequency selection network: LC oscillators, RC oscillators and quartzcrystal oscillators. Quartz crystal oscillators have high frequency stability and are only used in situations with high requirements. In general household appliances, various LC oscillators and RC oscillators are widely used.

LC Oscillator

The frequency selection network of the LC oscillator is the LC resonant circuit. Their oscillation frequencies are all relatively high, and there are three common circuits. （1） Transformer feedback LC oscillator circuit （1） Transformer feedback LC oscillator circuit （1） Transformer feedback LC oscillator circuit Figure 1（a） is a transformer feedback LC oscillator circuit. Transistor VT is a common emitter amplifier. The primary of transformer T is an LC resonant circuit that plays a frequency selection role, and the secondary of transformer T provides a positive feedback signal to the amplifier input. When the power is turned on, a weak transient current appears in the LC loop, but only the current with the same frequency as the loop resonant frequency f0 can generate a higher voltage at both ends of the loop, and this voltage is sent back to the base of the transistor V through the coupling of the primary and secondary sides of the transformer L1 and L2. As can be seen from Figure 1 (b), as long as the connection method is correct, the feedback signal voltage is in the same phase as the input signal voltage, that is, it is positive feedback. Therefore, the oscillation of the circuit is rapidly strengthened and finally stabilized.

The characteristics of the transformer feedback LC oscillation circuit are: wide frequency range, easy to start oscillation, but low frequency stability. Its oscillation frequency is: f 0 =1 / 2π LC. It is often used to generate sine wave signals from tens of kilohertz to tens of megahertz.

(2) Inductor three-point oscillation circuit

Figure 2 (a) is another commonly used inductor three-point oscillation circuit. In the figure, inductors L1, L2 and capacitor C form a resonant circuit that plays a frequency selection role. The feedback voltage is taken from L2 and added to the base of transistor VT. As can be seen from Figure 2 (b), the input voltage and feedback voltage of the transistor are in phase, satisfying the phase balance condition, so the circuit can oscillate. Since the three poles of the transistor are connected to the three points of the inductor respectively, it is called an inductor three-point oscillation circuit.

The characteristics of the inductor three-point oscillation circuit are: wide frequency range, easy to oscillate, but the output contains more high-order modulation waves and the waveform is poor. Its oscillation frequency is: f 0 =1/2π LC, where L=L1 + L2 + 2M. It is often used to generate sine wave signals below tens of megahertz.

( 3 ) Capacitor three-point oscillation circuit

Another commonly used oscillation circuit is the capacitor three-point oscillation circuit, see Figure 3 (a). In the figure, the inductor L and the capacitors C1 and C2 form a resonant circuit that plays a frequency selection role. The feedback voltage is taken from the capacitor C2 and added to the base of the transistor VT. From Figure 3 (b), it can be seen that the input voltage and feedback voltage of the transistor are in phase, satisfying the phase balance condition, so the circuit can oscillate. Since the three poles of the transistor in the circuit are connected to the three points of the capacitors C1 and C2 respectively, it is called a capacitor three-point oscillation circuit. The characteristics of the capacitor three-point oscillation circuit are: high frequency stability, good output waveform, and the frequency can be as high as 100 MHz or more, but the frequency adjustment range is small, so it is suitable for a fixed frequency oscillator. Its oscillation frequency is: f 0 =1/2π LC , where C= C 1 C 2 C 1 +C 2 .

The amplifiers in the above three oscillation circuits all use common emitter circuits. The oscillator with common emitter connection has higher gain and is easy to start. The amplifier in the oscillation circuit can also be connected in the form of a common base circuit. The oscillation frequency of the oscillator with common base connection is relatively high, and the frequency stability is good.

RC Oscillator

The frequency selection network of the RC oscillator is an RC circuit, and their oscillation frequency is relatively low. There are two commonly used circuits.

(1) RC phase shift oscillator circuit

Figure 4 (a) is an RC phase shift oscillator circuit. The three RC networks in the circuit play the role of frequency selection and positive feedback at the same time. From the AC equivalent circuit of Figure 4 (b), we can see that because it is a single-stage common emitter amplifier circuit, the output voltage Uo of the transistor VT is 180° out of phase with the output voltage Ui. When the output voltage passes through the RC network and becomes the feedback voltage Uf and is sent to the input terminal, since the RC network only produces a 180° phase shift for the voltage of a certain frequency f0, only the signal voltage with a frequency of f0 is positive feedback and causes the circuit to oscillate. It can be seen that the RC network is both a frequency selection network and a part of the positive feedback circuit.

The characteristics of the RC phase shift oscillator circuit are: the circuit is simple and economical, but the stability is not high and the adjustment is not convenient. It is generally used as a fixed frequency oscillator and in occasions with less demanding requirements. Its oscillation frequency is: when the parameters of the 3-section RC network are the same: f 0 = 1 2π 6RC. The frequency is generally tens of kilohertz.

(2) RC bridge oscillator circuit

Figure 5 (a) is a common RC bridge oscillator circuit. The series-parallel circuit of R1C1 and R2C2 on the left side of the figure is its frequency selection network. This frequency selection network is also part of the positive feedback circuit. This frequency selection network has no phase shift (phase shift is 0°) for a signal voltage with a specific frequency of f0, and voltages of other frequencies have phase shifts of varying sizes. Since the amplifier has two stages, the feedback voltage Uf taken from the output of V2 is in phase with the input voltage of the amplifier (2-stage phase shift 360°=0°). Therefore, when the feedback voltage is sent back to the input of VT1 through the frequency selection network, only a voltage with a specific frequency of f0 can meet the phase balance condition and oscillate. It can be seen that the RC series-parallel circuit plays the role of frequency selection and positive feedback at the same time.17)] In fact, in order to improve the working quality of the oscillator, a series voltage negative feedback circuit composed of Rt and RE1 is added to the circuit. Among them, Rt is a thermistor with a negative temperature coefficient, which can stabilize the oscillation amplitude and reduce nonlinear distortion. From the equivalent circuit of Figure 5 (b), we can see that this oscillation circuit is a bridge circuit. R1C1, R2C2, Rt and RE1 are the four arms of the bridge respectively. The input and output of the amplifier are connected to the two diagonals of the bridge respectively, so it is called an RC bridge oscillation circuit.

The performance of the RC bridge oscillation circuit is better than that of the RC phase shift oscillation circuit. It has high stability, small nonlinear distortion, and easy frequency adjustment. Its oscillation frequency is: when R1=R2=R, C1=C2=C, f0 = 1 2πRC. Its frequency range is from 1 Hz to 1 MHz.